A fuel cell is composed primarily of a solid electrolyte membrane that conducts protons and a pair of electrodes including a catalyst between which the solid electrolyte membrane is interposed. Perfluorosulfonic acid membranes, such as Nafion (registered trademark, the same applies hereinafter), have been conventionally used as a solid electrolyte membrane. However, it is necessary to humidify a perfluorosulfonic acid membrane for increasing the proton conductivity of the perfluorosulfonic acid membrane. Accordingly, a humidifying system needs to be used when electric power is generated using such a fuel cell. Installation of the humidifying system disadvantageously increases the size of the entire fuel cell apparatus, the amount of time and effort required, and the maintenance costs. Under these circumstances, power generation in a non-humidified system has been studied. A polybenzimidazole/phosphoric acid mixed system has been developed as a non-humidified proton-conductive membrane (e.g., see PTL 1 and NPL 1). In this system, imidazole units and phosphate ions serve as temporal proton carriers.